It is frequently necessary to supply a modulation signal to a microwave circuit, either to modulate the microwave signal traveling on the microwave circuit with another, lower frequency signal, or to turn on or turn off the transmission of microwave energy in the microwave circuit.
The source of modulation or bias signal cannot be connected directly to the microwave circuit (termed a transmission line) by conventional isolating elements such as a diode because a diode does not provide isolation between the transmission line and the bias source at microwave frequencies. It is conventional to use an inductor or an inductor together with a capacitor, the combination constituting a low pass filter (LPF) which transmits the lower frequency bias signals, but isolates the microwave signals from the bias source.
While an LPF is able to provide a high impedance to microwave signals and a low impedance to the bias signal, it has a limited operating bandwidth and thus cannot provide isolation over a broad range of microwave frequencies.
Specifically, although the LPF will operate satisfactorily at a range of frequencies from its cutoff frequency to a frequency somewhat above cutoff, for frequencies appreciably above cutoff the inductor or inductors in the LPF will be self-resonant, thereby effectively destroying the isolating properties of the filter at these frequencies.
Accordingly, one object of the present invention is to provide a microwave bias network which is operable effectively over a very wide bandwidth, i.e. provides isolation to a very broad range of microwave frequencies. Other objects are to provide a microwave bias network which is simple in construction, economical, and highly reliable. Further objects and advantages will become apparent from a consideration of the ensuing description.